1. Field of the Invention
The present invention relates to aeration systems and, more particularly, systems for aerating effluent in wastewater treatment plants, or the like.
2. Description of the Prior Art
In wastewater treatment plants, it is known to aerate effluent or sludge as part of the wastewater purification process commonly known as the xe2x80x9cactivated sludgexe2x80x9d process. This process typically includes a biological treatment system consisting of a contained volume of wastewater with an aerobic microorganism colony which thrives on biodegradation of the wastewater in an aerobic environment. In order to maintain the requisite aerobic environment, supplemental air must be introduced into the system to promote the dissolution of gaseous oxygen into the liquid phase. The introduction of supplemental air is commonly achieved through the use of either mechanical surface aerators or submerged diffused aerators.
The rate of dissolution of gaseous oxygen into the liquid phase is directly proportional to (1) the surface area of the gas bubbles produced by these aerators, wherein the oxygen molecule travels across this xe2x80x9cboundaryxe2x80x9d from the gaseous phase within the bubble to the liquid phase without the bubble; and (2) the residence time, i.e., the length of time the gas bubble is exposed to the liquid. For example, several hundred smaller gas bubbles contain more surface area than one large bubble containing the same total volume of air, and hence, the smaller bubbles provide more efficient oxygen transfer. Due to friction losses, however, the energy required to produce fine bubbles increases as the size of the diffuser orifices decreases. As a result, fine bubble diffusers which provide more efficient oxygen transfer, have high operating power costs, while larger bubble jet diffusers, i.e., xe2x80x9ccoarse bubble jet diffusersxe2x80x9d, which produce a larger bubble size with less oxygen transfer efficiency, have a lower operating power cost.
The coarse bubble diffuser offers the advantage of higher exit velocity which provides greater mixing activity within the aeration tank, resulting in increased residence time of the air in solution. This, in turn, provides an increase in the total oxygen transferred while increasing the oxygen transfer efficiency closer to that of a fine bubble system. Complete mixing of the microorganism colony within the aeration tank system is critical to ensure adequate waste treatment within the aeration tank. Inadequate mixing can result in portions of the microorganism colony to settle to the tank bottom thereby limiting exposure of the microorganisms to the wastewater.
It is an object of the present invention to provide a diffused aeration system with improved overall oxygen transfer efficiency with little or no concomitant increase in operating power cost.
It is a further object of the present invention to provide a diffused aeration system having improved oxygen transfer efficiency while providing improved mixing capacity.
The above objects of the invention are satisfied with an improved aeration system which, in accordance with the present invention, is comprised of a plurality of coarse bubble jet diffusers connected with an air supply and a plurality of fine bubble jet diffusers connected with an air supply and disposed between the coarse bubble jet diffusers. Exemplary of suitable coarse bubble jet diffusers are the duckbill check valves described in U.S. Pat. No. 4,607,663, incorporated herein by reference. Preferred are the rubber duckbill check valves commercially available as TIDEFLEX(copyright) valves from Red Valve Company, Inc. of Carnegie, Pa. Exemplary of suitable fine bubble jet diffusers are those commercially available as DOMEFLEX(trademark) diffusers, also from Red Valve Company, Inc.
Each of the coarse bubble jet diffusers is connected with the air supply by at least one, preferably two, outer conduits. Likewise, each of the fine bubble jet diffusers is connected with the air supply by at least one inner conduit. In a preferred embodiment, the improved aeration system of the present invention comprises an aeration tank which contains a plurality of peripheral coarse bubble jet diffusers with a plurality of fine bubble jet diffusers located therebetween. The coarse bubble jet diffusers are connected with parallel, outer air distribution header pipes, and the fine bubble jet diffusers are connected with parallel inner air distribution header pipes. The inner air distribution header pipes are parallel to each other and to the outer header pipes. All header pipes are connected with an external air supply.
The coarse bubble jet diffusers are oriented to produce a high velocity air flow upward along opposing tank walls and across the tank surface such that the current from outer headers converges in the middle and is drawn back toward tank bottom due to the inductive downward flow streams created by the coarse bubble jet diffusers. The fine bubble jet diffusers are strategically located within the path of the inductive downward flow streams such that fine bubbles emitted by these diffusers travel in a flow path which is counter-current to the inductive downward stream created by the forced perimeter flow of the coarse bubbles. In operation, the diffused aeration system of the invention maximizes oxygen transfer efficiency, and maximizes mixing with little or no increase in operating power costs.